Colors displayed on a display consist of components of red, green and blue and called an RGB format on the basis of the respective initial letters. In contrast, colors used for printing by a printer are mainly consisting of color components of cyan, magenta, yellow and black, and are called a CMYK format also on the basis of the respective initial letters. Where colors displayed on a display are printed by a printer, a color conversion processing is needed, by which colors expressed by the RGB format are converted to those expressed by the CMYK format.
In theory, a color conversion processing from the RGB format to the CMYK format can be performed by a uniform formula. However, in general, the uniform formula can not provide satisfactory results for all colors, due to a difference in quality of ink or toner.
Therefore, a tool is used, which is called Lookup Table (hereinafter abbreviated as LUT). This is a table tabulated on the basis of a rule that predetermined color-component values of CMYK are to be used in accordance with predetermined color-component values of RGB. Where color-component values of RGB are given, for example, values from 0 to 255, respectively, approximately 16,770,000 combinations (256×256×256) of those color are available. An optimal color-component value of CMYK is given to each of these combinations. However, the combinations are available in such a larger number that it is difficult to store all data for the LUT due to a limited capacity, and it is also extremely troublesome to prepare the LUT.
Therefore, such a LUT is used in reality that certain representative points are selected from 256 stages of color-component values of RGB to arrange color-component values of CMYK corresponding to the representative points. The color-component values of RGB available at the representative points can obtain values of CMYK with reference to the LUT, directly. In contrast, for the color-component values of RGB not available at the representative points, approximate values must be calculated with reference to the representative points in the vicinity.
Methods for calculating the approximate value include, for example, a triangular pyramid interpolation (4-point interpolation), a triangular prism interpolation (6-point interpolation) and a rectangular interpolation (8-point interpolation). In these methods, several representative points in the vicinity of the color-component values of RGB that have been input are selected to calculate an approximate values with reference to values of the LUT at the representative points. In calculating the approximate value, addition and multiplication are repeatedly performed and division is finally performed with reference to the values of the LUT, as disclosed in JP-A-8-307684.